Semiconductor integrated circuits are typically manufactured using a lithographic process. Lithography may involve, e.g., coating a semiconductor wafer with a photosensitive resist, projecting light through a patterned mask onto the resist, and developing the exposed resist. The wavelength of light used in the lithography process is a key factor in the drive for higher levels of microcircuit integration. Since the minimum processing dimension of lithography depends on the wavelength of light used, it is necessary to shorten the wavelength of the irradiated light in order to improve the integration degree of the integrated circuit. In recent years, extreme ultraviolet (EUV) radiation which radiates extreme ultraviolet radiation with wavelengths from 13 nm to 14 nm, has been developed as semiconductor lithography light source to meet the demands for micro-miniaturization of semiconductor device.
There are a number of methods of generating EUV radiation. In one example, EUV radiation may be generated through plasma in which high temperature plasma is first created by heating and excitation of an extreme ultraviolet radiating species and then the EUV radiation radiated from the plasma is extracted. However, both higher-harmonic generation as well as thermally produced plasma processes require very high peak power. In addition, the laser produced plasma EUV light source has a relatively low repetition rate.
Another method of generating EUV radiation is free electron laser (FEL). A FEL involves interaction between a high brightness electron beam and an intense light beam while traveling through a periodic magnetic field to generate coherent electromagnetic radiation. Specifically, an electron beam is first accelerated to almost the speed of light with very high kinetic energies from about 100 MeV to 1 GeV. The accelerated beam in turn passes through a FEL oscillator, a periodic transverse magnetic field produced by an array of magnets with alternating poles within an optical cavity along the beam path. The acceleration of the electrons along this path results in the release of photons, which, with appropriate optical system, may emit a coherent light beam of extremely high power. The optical system typically includes a ring resonator having multiple mirrors. While these have proven effective for wavelengths ranging from the far IR to the UV, it becomes difficult to implement FEL for EUV generation because the reflectivity of metals and other mirror coatings drops significantly at shorter wavelengths and thus lack of good reflecting surfaces to form the mirrors.
Another FEL method involves a process of self-amplified spontaneous emission (SASE). These FELs do not use resonator mirrors and may operate at short wavelengths on a single pass of a high brightness electron through a long undulator. In particular, all electrons are initially distributed randomly and emit their incoherent spontaneous radiation. Through the interaction of their radiation and oscillations of electrons, they drift into microbunches separated by a distance equal to one radiation wavelength. Through this interaction, all electrons begin emitting coherent radiation in phase. However, SASE requires a very bright electron beam (i.e., high peak current, low emittance and small energy spread) and a comparatively long undulator to build up beam intensity from spontaneous noise to a saturated intensity.
It is within this context that aspects of the present disclosure arise.